A structure in which a protective film is laminated on one or both surfaces of a polarizer formed with a polyvinyl alcohol (hereinafter, referred to ‘PVA’)-based resin using an adhesive have been commonly used. Triacetyl cellulose (TAC)-based films have been normally used as a polarizing plate protective film in the art, however, such TAC films have a problem of being readily deformed in high temperature and high humidity environments. Accordingly, protective films made of various materials capable of replacing TAC films have been recently developed, and for example, a method of using polyethylene terephthalate (PET), a cycloolefin polymer (COP), and an acryl-based film either alone or as a mixture thereof has been proposed.
Herein, aqueous adhesives formed with an aqueous solution of a polyvinyl alcohol-based resin are normally used as an adhesive used to attach the polarizer and the protective film. However, aqueous adhesives have a problem in that the use is limited depending on the material of a film, since adhesive strength is weak when acryl-based films or COP films and the like are used as the protective film instead of TAC. In addition to the problem of adhesive strength defects depending on the materials, the aqueous adhesive also has problems in that curls are generated in a polarizing plate due to a drying process of the aqueous adhesive, and initial optical properties are degraded when materials of the protective film used on both surfaces of a PVA element are different. Moreover, a drying process is absolutely required when the aqueous adhesive is used, and differences in moisture permeability, heat expansion and the like occur in the drying process leading to a problem of a defect rate increase. As an alternative to solve the problems described above, methods of using non-aqueous adhesives instead of aqueous adhesives have been proposed.
Meanwhile, non-aqueous adhesives for a polarizing plate that have been proposed so far mostly use ultraviolet-curable adhesives curing adhesive compositions by UV irradiation, and most of these ultraviolet-curable adhesives secure adhesive strength through hydrogen bonding between a hydroxyl group of a polarizer formed with a PVA-based resin and a hydrophilic functional group of an ultraviolet-curable adhesive. However, when hydrogen bonding is used as described above, water molecules are also capable of hydrogen bonding with a hydroxyl group of the polarizer formed with a PVA-based resin or a hydrophilic functional group of the ultraviolet-curable adhesive under a high humidity atmosphere, therefore, there is a disadvantage in that the number of hydrogen bonding between a hydroxyl group of the polarizer formed with a PVA-based resin and a hydrophilic functional group of the ultraviolet-curable adhesive is reduced, which leads to the reduction of adhesive strength.
In view of the above, researches on securing adhesive strength through covalent bonding instead of hydrogen bonding have been actively conducted, and among these, researches particularly on ultraviolet-curable adhesives including an epoxy compound have been most active. In the case of ultraviolet-curable adhesives including an epoxy compound, a ring-opening reaction between an epoxy compound and a hydroxyl group of a polarizer formed with a PVA-based resin occurs during the curing by ultraviolet radiation, and covalent bonding is produced by the ring-opening reaction, and therefore, excellent adhesive strength may be secured even under a high humidity atmosphere. However, curing of such ultraviolet-curable adhesives including an epoxy compound is progressed using a cation method rather than using a radical method, therefore, there are many disadvantages in the manufacturing process due to a low curing speed and a low degree of curing.
Accordingly, radical-curable adhesives having a high curing speed and a high degree of curing by progressing the curing using a radical method, and capable of securing adhesive strength through covalent bonding instead of hydrogen bonding thereby having excellent water resistance have been required.
Meanwhile, in another aspect, display devices having a structure in which a polarizing plate is attached to an upper and/or a lower substrate of a display panel that generates images using an adhesive have been generally used, and in order to obtain thin display devices, a method of laminating a protective film on only one surface of a polarizer using an adhesive, and directly attaching the surface of the polarizer opposite to the protective film-laminated surface to a display panel through an adhesive as a medium without a protective film has been proposed.
Herein, acryl-based adhesives are normally used as an adhesive used to attach a polarizing plate having the structure described above to a display panel. However, as for the acryl-based adhesive, at least a thickness of 20 μm is commonly required in order to maintain proper adhesive strength, and therefore, there is a problem in that the trend of display devices being thinner and lighter is not satisfied. In addition, in the case of acryl-based adhesives, an adhesive layer is generally formed using a method of applying an adhesive composition on a releasing film, drying the solvent, and then transferring the result on a sample surface, and this method has disadvantages in that the method is inconvenient when attaching a polarizing plate to a display device, and productivity decreases. Particularly, the acryl-based adhesive commonly has a glass transition temperature of 0° C. or less, and when this adhesive is directly attached to a polarizer and used, there is a problem in that heat resistance reliability is reduced, such that cracks occur in the polarizer in a thermal shock reliability test.
Accordingly, new attaching means capable of being formed to a thin film, improving productivity when attaching a polarizing plate and a display panel, and having superior heat resistance reliability has been required.